FIGURE 3.
(A and B) The boundaries of a cell as it spreads after activation by either addition of fMLF into the bulk (A) or by presentation of a gradient in fMLF (B). The red closed lines represent cell boundary profiles separated in time by either 2 s (A) or 4 s (B). The direction of the fMLF gradient is shown as a thick black arrow in the right-hand corner of B. Note that as the cell spreads, one edge of the contour hardly moves (indicated by curved arrows bracketing the pinned zone). The trajectory of the centroid is shown as a thin black line inside the cell boundaries. The shape of the cell boundary just after addition of fMLF is shown as a blue dotted line, the shape just before beginning of migration as a broken black line, and the shape corresponding to the appearance of the first point of close persistent contact with the substrate as a solid gray line. In both cases, the approximate position of the first point of contact is indicated by a black filled oval, and the eventual direction of migration as a gray filled arrow next to the trajectory of the centroid. Note that in the case of uniform stimulation, the first point of close contact is roughly in the zone that later becomes the lamellipod, whereas in the case of a gradient in the concentration of fMLF, the first point of contact is in a zone that later becomes the uropod. (C and D) Close-up of the trajectory for initial few points corresponding to the cells depicted in A and B, respectively. The first few points are marked with numbers to show that the centroid executes a random motion. Only later does the walk acquire a directionality that is indicated by a black arrow; corresponding arrows are marked in A and B.